2/16/2012 6:00 AM PST

Bobby Brown is about to make a money grab by leeching onto his daughter, the sole heir to WhitneyHouston's fortune -- at least that's what Whitney's mom and other family members believe.

Family sources tell TMZ ... Cissy Houston is aggressively trying to block Bobby from getting close to 18-year-old Bobbi Kristina, fearing it's just a ploy to line his pockets.

The family is convinced ... Bobby is broke, and he's making a play for his daughter's affection to latch on to what could be an estate worth around $20 million, and that figure could double by the end of this year.

We're told friends of the Houston family have already contacted Bobby and warned him he is persona non grata at Whitney's funeral and in Bobbi Kristina's life.

There's a lot of bad blood between Whitney's family and Bobby Brown, because they believe he was the driving negative force in her life that led her down the substance abuse path. They also believe he's largely responsible for Bobbi Kristina's drug and alcohol struggles.

As for Bobby Brown ... people in his camp tell TMZ, he's well aware of the family's animosity toward him and wants to bury the hatchet for the sake of his daughter.